Glass fibers used for the transmission of light signals have a cylindrical structure in which the index of refraction of the center of said structure (for example, a homogenous core) is greater than the index of refraction of the periphery (for example, a sheath). The numerical aperture of such a fiber increases with increasing divergence between the indices of refraction of the center and periphery.
The difference between the coefficients of expansion of the glasses constituting the core and sheath should be slight enough to avoid the formation of stresses which can cause fractures during the fabrication of the semifinished product and during its drawing into the shape of rods and, ultimately, into fibers.
It is known in this field that optical fibers should have the lowest possible losses from absorption and diffusion and the best glasses are pure silica and various doped silicas. It is also known that the index of refraction of the silica can be modified by addition of doping elements with titanium, aluminum, or germanium oxides increasing the index of refraction and boron and fluorine decreasing it.
Of the various possible combinations, the one that best meets these requirements is SiO.sub.2.TiO.sub.2 /SiO.sub.2.F.
To produce an SiO.sub.2.TiO.sub.2 /SiO.sub.2.F composite structure having a very good transparency and a large numerical aperture by drawing, however, it is necessary to solve two major difficulties. First, it is known that the Ti.sup.3+ ion considerably increases the light losses by absorption as indicated particularly by French Pat. No. 2,002,589. Hence, it is necessary to prevent even the very slightest part of the titanium from going from the Ti.sup.4+ oxidation state to the Ti.sup.3+ state during formation of the titanium-doped silica, during its heating when a second glass is deposited, and during its drawing. Second, a sufficient amount of fluorine must be incorporated in the silica to reduce the index of refraction of said silica significantly.
It has long been known how to prepare a titanium-doped silica by thermal decomposition of Si and Ti gaseous compounds, such as SiCl.sub.4 and TiCl.sub.4 in the presence of oxygen. However, the standard process using an oxyhydrogen blower gives a silica containing an average of 1000 parts per million (ppm) of hydroxyl groups. This high OH ion concentration produces intense absorption bands in the near infrared spectral zone that are widely used in industrial optical application. Such concentrations, however, are not suitable for use in telecommunication optical fibers. The influence of hydroxyl groups on attenuation in pure silica or simple silica-base glasses is described, for example, in Kaiser et al., Journal of the Optical Society of America, Vol. 63, No. 9, p. 1141 (1973). According to this article, for an OH ion content of 50 ppm, the attenuations measured at wavelengths of 720, 820, 880 and 945 nanometers are 3.5; 0.2; 4.5 and 50 dB/km, respectively.
To remedy this drawback, an effort has been made to exclude any trace of water in the silica. Such a process is described in French Pat. No. 1,380,371. It constitutes introducing a hydrogen-free, oxidizable silicon compound in a flame that is also hydrogen-free.
This type of process was then used to obtain a titanium-doped silica. As disclosed in French Pat. No. 2,150,327, at least a hydrogen-free, oxidizable silicon compound and a hydrogen-free, oxidizable titanium compound may be introduced into a gaseous current, also hydrogen-free, containing oxygen, brought to a high temperature. Under these conditions, titanium-doped silica, free of OH ions is obtained. Nevertheless, such silica has a very slight transparency because of a violet coloring arising from the incorporation in the silica of titanium partly in Ti.sup.3+ form.
It is also known to make fluorine-doped silica and cover a siliceous material with it. Thus, French Pat. No. 2,208,127 describes the deposit of a fluorine-doped silica glass on a rod or tube of pure molten silica. A rod driven in a double translation and rotation movement is subjected to an atmosphere of gaseous silicon fluoride SiF.sub.4. By oxidation in a plasma, silica is formed in which fluorine is incorporated. However, the process makes it possible to introduce only slight amounts of fluorine in the silica layer that is formed, and the divergence between the indices of refraction is insufficient to obtain the desired optical characteristics in the glass fiber.
To remedy this drawback, French Pat. No. 2,321,459 describes a process of preparing vitreous silica, doped with fluorine and free of OH ions, by reaction of a silicon compound, such as SiCl.sub.4, and a fluorine compound, with the oxygen contained in a hydrogen-free gaseous current in the flame of an induction plasma burner. The compound used to dope the silica is a fluorocarbon compound, namely, dichlorodifluoromethane, CCl.sub.2 F.sub.2, added in vapor form to the oxygen introduced in the plasma burner. This compound decomposes in the very hot flame of the plasma at the same time that SiO.sub.2 is formed. The vitreous silica, thus doped with fluorine, is deposited radially on the surface of a cylindrical blank of pure silica or one doped with metal oxides.
This process indeed makes it possible to obtain a sufficient amount of fluorine in the silica formed on the surface of the blank but it has a major drawback. It does not teach how to obtain in the center of the blank a titanium-doped silica free of Ti.sup.3+ ions. Further, the presence of carbon in the molecules of the fluorine compound increases the risk of reduction of the titanium which would increase still more the proportion of Ti.sup.3+. This process therefore does not seem suited to obtain optical fibers of suitable quality.
French Pat. No. 2,321,710 describes another method for obtaining an optical fiber consisting of a titanium-doped silica core and a fluorine-doped silica sheath. It consists in starting with a fluorine-doped silica cylinder in which a titanium-doped silica rod is introduced and in thoroughly fusing these two pieces by drawing them in a tubular furnace.
Besides the drawbacks already mentioned, this process requires a series of machining, polishing and cleaning operations that make it long and costly. Further, there is the danger that defects, which are a considerable cause of diffusion losses, will appear at the core-sheath interface.